Why does NSManagedObject not get inserted into a child NSManagedObjectContext when the context initialised inline?

ChildContextInsertion.swift

import CoreData
import PlaygroundSupport

extension NSManagedObjectContext {
    func childContext(concurrencyType: NSManagedObjectContextConcurrencyType = .mainQueueConcurrencyType) -> NSManagedObjectContext {
        let childContext = NSManagedObjectContext(concurrencyType: concurrencyType)
        childContext.parent = self
        return childContext
    }
}

// Requires a momd called "books" be added to the Resources directory of the playground
let container = NSPersistentContainer(name: "books")
container.loadPersistentStores{ description,error in
    
    let childContext = container.viewContext.childContext()
    let obj1 = NSEntityDescription.insertNewObject(forEntityName: "Book", into: childContext)
    print(obj1.managedObjectContext == nil)
    
    let obj2 = NSEntityDescription.insertNewObject(forEntityName: "Book", into: container.viewContext.childContext())
    print(obj2.managedObjectContext == nil) // prints true: why?
    
}

PlaygroundPage.current.needsIndefiniteExecution = true